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Constraining Populations of Extragalactic Gamma-ray Sources with

the Extragalactic Gamma-ray Background

Vasiliki PavlidouUniversity of Chicago

Tonia Venters University of Chicago and APC (Paris 7)

Jennifer Siegal-Gaskins University of Chicago

Brian Fields University of Illinois

Angela Olinto University of Chicago and APC (Paris 7)

Carolyn Brown University of Chicago

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Extragalactic Gamma-ray Emitters:1. Classes with already detected members

• Blazars: – AGNs with jet aligned with line-of-sight– Most populated class of EGRET identified sources – Only class with identified members at high z

• Normal galaxies:– EGRET detected only 2: Milky Way, LMC– GLAST will detect only 3 more: SMC, M31,

M33?

• Unidentified EGRET sources:– Likely to include many extragalactic sources– More numerous than blazars!

Credit: J. Buckley 1998 (Science),illustration: K. SutliffCredit: S. Digel

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• Starburst galaxies– GLAST will likely detect ~10 (Torres et a 2004)

• Merging / accreting clusters of galaxies and large-scale filaments

• Dark matter density peaks

• Insert your own here

Extragalactic Gamma-ray Emitters:2. Theorized but yet-undetected classes

Credit: B. Whitmore (HST)

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• Theorize, then theorize some more.

• Use observations of the isotropic diffuse background to constraint properties of yet-undetected classes / exotic physics

• However: – unresolved members of established classes (blazars, normal galaxies,

extragalactic unidetified sources) also have some guaranteed contribution to background

– These contributions need to be modeled and subtracted

– Task involves formidable uncertainties!

How do we make progress on extragalactic source classes with no identified members?

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• Use information on background spectral shape

How do we make progress on extragalactic source classes with no identified members?

Observational EGRB(Sreekumar et al 1998)

Observational EGRB(Strong et al 2004)

Strong et al 2004 systematics

Unresolved blazars(VP & Venters 2007)

Unresolved normal galaxies(VP & Fields 2002)

Unresolved extragalactic unidentified sources(VP, Siegal-Gaskins, Fields, Olinto, & Brown 2007)

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How confident are we in the spectra of unresolved emission from established source classes?

• Unresolved emission spectra: – calculated from spectral index

distributions of detected sources

– implicit assumption: spectra of unresolved sources similar to spectra of resolved sources

– but also: uncertainty involved in spectral index distribution of resolved sources (low number statistics, large measurement error in individual spectral indices)

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are needed to see this picture.

EGRET constraints on the unresolved blazar emission spectral shape (VP & Venters 2007)

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GLAST can help!

• GLAST will detect between 1,000 - 10,000 blazars (EGRET detected ~ 60)

• v

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are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Unresolved FSRQ emission using EGRET data

Unresolved FSRQ emission using GLAST data (simulated)

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• Extragalactic diffuse emission encodes valuable information on (a) unresolved members of established classes(b) theorized, yet-undetected classes of extragalactic gamma-ray sources

• Spectral shapes of different components / observed emission powerful tool: * complementary to studies of relative emission intensity * involves different, independent uncertainties

• GLAST will allow us to make much more confident predictions for spectral shape of known classes, and place much stricter constraints on properties of undetected classes

Conclusions